This invention relates to refractive eye surgery and especially to refractive eye surgery using a random scanning of one or more laser beams in the ablation of cornea tissue to reshape the cornea of an eye.
The cornea is a thin shell with nearly concentric anterior and posterior surfaces and a central thickness of about 520 micrometers. It has an index of refraction of 1.377 and a nominal radius of curvature of 7.86 mm. The epithelium, forming the anterior surface of the cornea, is about 70 micrometers thick in young people at the center. Underlying the epithelium is a layer called Bowman's layer or Bowman's membrane, which is about 12 micrometers thick. This covers the anterior surface of the stroma, which makes up the bulk of the cornea and consists primarily of collagen fibers. The endothelium forms the posterior layer of the cornea and is a single layer of cells.
About three-quarters of the refractive power of the eye is determined by the curvature of the anterior surface of the cornea, so that changing the shape of the cornea offers a way to significantly reduce or eliminate a refractive error of the eye. The stroma is thick enough so that portions of its anterior region can be ablated away to change its profile and thus change the refractive power of the eye for corrective purposes, while leaving plenty of remaining stroma tissue.
Various lasers have been used for ophthalmic applications including the treatments of glaucoma, cataract and refractive surgery. For refractive surgeries (or corneal reshaping), ultraviolet (UV) lasers, such as excimer lasers at 193 nm and fifth-harmonic Nd:YAG at 213 nm have been used for large area surface corneal ablation in a process called photorefractive keratectomy (PRK). Corneal reshaping may also be performed by laser thermal coagulation currently conducted with Ho:YAG lasers using a fiber-coupled, contact and non-contact type processes.
Refractive surgery has reached a new dimension due to the development of the excimer laser (193 nm) and fifth harmonic solid state laser (190 nm-215 nm) being used to photoablate the cornea tissue to reshape the cornea. Several approaches have been proposed to deliver the laser beams to the surface of the cornea including using a mask or diaphragm and move the mask or diaphragm to block the laser beam to achieve a desired curvature on the outer surface of the cornea. It has also been proposed to use a scanner to move a laser beam spot on the outer surface of the cornea to ablate the tissue to change the curvature on the cornea. Combining the mask or diaphragm and scanner to block and move a laser beam is also used to achieve a desired curvature on the outer surface of the cornea. The mask or diaphragm approach requires a high energy laser and a rough or stepped cornea surface is generated in the laser interacting with the cornea. When the laser interacts with the corneal tissue, it generates some water that remains on the surface of the cornea (like sweat water). This changes the ablation rate when a new laser pulse reaches the cornea. If this is not taken into consideration, an irregular pattern can be induced called an "island". Central corneal islands have been described in connection with prior laser beam delivery systems.
The scanning or combination of mask and scanner approach produces a smoother cornea surface but nonsymmetrical beam profiles and the sweat water effect creates an island effect which is caused by a nonsymmetrical ablation on each side or point of the corneal surface.
The present invention may use one, two or more laser beams with the plural beams being formed by splitting one laser beam and uses a random scanning pattern of the beams to scan the lasers over the cornea. The spatial energy distribution mode is scanned on the cornea or in the cornea simultaneously by using one or more scanning devices controlled by a predetermined program in a computer controller. The computer generates a random scanning pattern from a database of non-random scanning points preselected for the area to be allotted. Because the laser beam is located and moved on the cornea, the cornea will compensate for the uneven situation of the sweat water effect when the laser interacts with the cornea tissue and non-symmetrical laser beam spatial energy distribution.
Refractive error can be divided in two categories, spherical and cylindrical. Spherical error can effect the eye vision as myopic or hyperopic. Cylindrical error can effect the eye vision as myopic or hyperopic astigmatism. The present invention also uses a computer program to avoid ablation of the central part of the cornea in hyperopic astigmatism which results in a safer, more predictable, and faster surgery procedure. In the case of hyperopic combined with astigmatism of a cornea, the center is not ablated.